Using a sharp paring knife, make a lengthwise slit down one side of each banana, slicing through the peel but not the flesh and leaving the ends intact. Loosen peel slightly with your fingers. Stuff each banana with 1/4 cup marshmallows and 2 tablespoons of chocolate chips. Press peel together gently to enclose filling completly, wrap each banana in foil.
 

Heat a grill to low heat. Place banana packets on grill; cook, turning occasionally, until they are softened and filling has slightly melted, 5 to 7 minutes, depending on the ripeness of the banana. Remove bananas from grill, and let cool slightly before removing foil. Serve.